The CDC is investigating hepatitis A outbreaks across six states among patients reporting drug use and/or homelessness and their contacts, the agency stated in a recent advisory.

Since January 2017, California, Indiana, Kentucky, Michigan, Utah and West Virginia have reported a total of more than 2,500 infections.

The outbreak is the largest since the agency recommended vaccinations for at-risk populations in 1996. It is also the first large outbreak among homeless Americans, the CDC stated.

The CDC is advising healthcare providers to consider hepatitis A in any patients with jaundice and clinically compatible symptoms. The agency also encourages vaccination for at-risk populations as soon as possible, within two weeks of exposure to the virus. All diagnosed hepatitis A cases should be reported to the public health department.

The CDC is continuing to monitor demand for the adult hepatitis A vaccine, which faced some supply constraints but is now more available.